The introduction in recent years of optical amplifiers with wide wavelength operating range in the 1520 to 1580 nm spectral region into fiber communication systems has enabled the practice of dense wavelength division multiplexing (DWDM). In DWDM systems a group of sixteen or even as many as sixty four wavelengths are simultaneously transmitted in a fiber, each wavelength being modulated by a data stream as fast as 10 Gb/s. Such high capacity communication systems consist of optical transmitters, cabled fiber, optical amplifiers, wavelength multiplexers and optical receivers and need to be closely monitored to detect any malfunction that may corrupt the information transmission. The bit error rate (BER) is a measure defined as the number of errored bits divided by the total number of bits received in a given time period. The BER is sensitive to several parameters of the transmission system, such as the optical power at the receiver, the quality of the transmitter, but particularly to the ratio of signal power to noise power, known as the signal to noise ratio (SNR) at the receiver. In turn, the SNR is determined by the addition of receiver thermal noise shot noise and noise added by optical amplifiers in the system.
One of the most important parameters of optical amplifiers is the gain at the various wavelengths. For proper operation the receivers operating at the various wavelengths expect a common and substantially equal optical signal to noise ratio as well as substantially equal optical power. Since transmitters generally output substantially equal amounts of power at various wavelengths, the amplifiers in the system are expected to provide equal gain at the various channel wavelengths.
Several methods of equalizing or flattening the gain of optical amplifiers have been explored in the literature. The book titled "Erbium Doped Optical Amplifiers" by Emmanuel Desurvire discusses such gain flattening techniques on pp. 480. The article, "Flat-gain amplifiers and transmission in WDM networks," paper FA1-1 presented at the "Optical amplifiers and their applications" conference by Bertrand Clesca also compares gain flattening techniques. In Erbium doped fibers the gain at any given wavelength has contributions from several broadened atomic resonances. Each of these resonances is centered at a slightly different wavelength in the 1520 to 1580 nm range and originates from a different pair of Stark sublevels of the Erbium ion electronic energy levels. The material of the glass matrix containing the Erbium, as well as dopants in that matrix, affect the position and broadening of the atomic resonances in Erbium. One of the known techniques for flattening the gain curve is the use of aluminum co-doping of Erbium doped fiber. Another technique utilizes fluoride glass instead of silica glass as the fiber material. The use of fluorozirconate glasses for example is well known to yield flatter gain over wavelength when doped with Erbium. Fluorozirconate based doped fibers are available from Galileo Corporation of Sturbridge, Mass. The drawback of fluoride fibers has been their low melting point, making them very difficult to manufacture and splice with other fibers in a system and their hygroscopic nature, making them susceptible to damage by humidity.
Yet other techniques insert specially shaped spectral filters in line with the amplifiers to compensate for the difference in gain at different wavelengths. U.S. Pat. No. 5,050,949, by Di Giovanni and Giles, describes the use of two stage fiber amplifiers to achieve flattened gain. The drawback of the two stage approach is that it still lacks enough suppression of the gain in the 1520 to 1535 nm spectral region to achieve the desired degree of flatness.
More recently, U.S. Pat. No. 5,557,442 by Huber describes a technique involving the use of a circulator, a series of fiber Bragg reflectors imprinted into a fiber and a set of attenuators to achieve gain flattening. Circulators are optical devices with three or more fiber ports that channel the light from port I into port I+1. Bragg gratings are periodic index of refraction gratings imprinted into the fiber core by UV light and reflect fiber propagating light at specific wavelengths matching the periodicity of the gratings. In U.S. Pat. No. 5,557,442, the light enters the amplifier, then the circulator, and is channeled into the chain of Bragg reflectors and attenuators. The light at a first wavelength w.sub.1, which experiences little gain in the amplifier, is reflected from the first Bragg grating and thus suffers no attenuation before returning to the circulator and being channeled out. Light at a second wavelength w.sub.2, which experiences somewhat higher gain than w.sub.1, is reflected by a subsequent Bragg grating and thus has to go through one or more attenuators before it returns to the circulator and gets channeled out. Light at a wavelength w.sub.3, experiencing the highest gain in the amplifier, is reflected back by the last Bragg grating in the fiber, thus forcing it to go through the whole attenuator chain twice before it reaches the circulator and gets channeled out.
The main drawback of the Huber patent is that it requires complex variable attenuators to adjust the amount of loss dynamically to correct for changes in the gain of the amplifier that are due to input power changes. Other dynamic methods of controlling the gain which are responsive to variations in the input power to the amplifier have also been proposed in the literature. U.S. Pat. No. 5,088,095 to Zirngibl for example teaches the use of a counter propagating light beam to clamp the gain. However, this method reduces the useful power available from the amplifier due to the transfer of power to the back propagating light beam and achieves little flattening of the gain.
It is therefore apparent from examining the prior art that a method of flattening the amplifier gain over wavelength that is responsive to variations in input power is needed.
A first objective of this invention is to provide a method of equalizing or flattening the gain of a doped fiber amplifier over wavelength without affecting the optical properties of the amplifier such as average gain and noise figure.
A second objective of the present invention is to flatten the gain of an amplifier independently of the input power level to the amplifier.